Since solid state light sources such as LEDs have advantages of, e.g., good purity of color of emitted light, compact size and excellent lifetime characteristic compared to high-pressure mercury lamps; recently, three panel-type projectors including solid state light sources have been provided.
For example, three panel-type liquid-crystal projectors including three light sources, a red LED, a green LED and a blue LED, have been known.
In such three panel-type liquid-crystal projectors, red light emitted from the red LED is applied to a liquid-crystal panel section for red, green light emitted from the green LED is applied to a liquid-crystal panel section for green, and blue light emitted from the blue LED is applied to a liquid-crystal panel section for blue. Then, the red light, the green light and the blue light that have passed through the respective liquid-crystal panel sections are combined by a color combining section, and the light of the respective colors combined by the color combining section is projected onto a screen via a projection lens.
Although LEDs have been enhanced in performance year after year, the luminous flux of light emitted from such LEDs is still small compared to that of discharge lamps, and thus, brightness of an image projected by a projector using LEDs is not sufficient. Here, a luminous flux represents brightness of the entire light radiated from a light source in a certain direction, and the unit of the luminous flux is lumen (lm). A large luminous flux is synonymous with a large amount of light.
As a first approach for obtaining a bright projection image, there is the method of increasing luminous fluxes of light emitted from light sources. More specifically, use of LEDs with a large light emission area or LEDs arranged in an array enables provision of light sources with a large luminous flux.
However, the above first approach has the following problem.
In general, the amount of light from a light source that can be used as projection light in a projector is determined by the relationship between the light source-side etendue defined by the product of the light emission area of the light source and the divergence angle and the image formation-side etendue defined by the product of the area of a liquid-crystal panel and the acceptance angle (solid angle) determined by the F number of a projection lens. In other words, in a projector, unless the value of the light source-side etendue is smaller than the value of the image formation-side etendue, light from the light source cannot efficiently be used as projection light.
Accordingly, even where the luminous flux of light emitted from a light source is increased by arranging LEDs in an array or using a LED with a large light emission area, the brightness of a projection image cannot be enhanced if the value of the light source-side etendue is larger than the value of the image formation-side etendue. Because of such etendue restriction, a projection image with sufficient brightness cannot be provided by merely increasing the luminous flux of light emitted from a light source under the current circumstances.
As a second approach for obtaining a bright projection image, there is the method of increasing the utilitization efficiency of light emitted from a light source. The method will briefly be described below.
In general, in a liquid-crystal projector, a liquid-crystal panel section includes a liquid-crystal panel and two polarizing plates arranged respectively on the incident surface side and the exit surface side of the liquid-crystal panel. In the liquid-crystal panel section, light that has passed through the incident surface-side polarizing plate becomes linearly-polarized light and the linearly-polarized light is entered onto the liquid-crystal panel. While the incident linearly-polarized light is propagating through the liquid-crystal layer of the liquid-crystal panel in the thickness direction of the liquid-crystal panel, the polarization state of the linearly-polarized light varies according to the refractive index anisotropy (birefringence) of the crystal. The exit surface-side polarizing plate transmits, of output light that has passed through the liquid-crystal layer, only polarized light in a particular direction.
In the above liquid-crystal panel section, only one type of polarized light (s-polarized light or p-polarized light) in the incident light is used and the other type of polarized light is absorbed or reflected and thus does not contribute to formation of an image. Thus, if incident light is non-polarized light, a light loss of approximately 50% will occur in the liquid-crystal panel section.
In the above three panel-type liquid-crystal projector, light of each of the colors emitted from the red LED, the green LED and the blue LED is non-polarized light, and thus, if the light emitted from each LED is entered onto the liquid-crystal panel section without any alteration, a light loss of approximately 50% will occur in the liquid-crystal panel section.
Therefore, the method (second approach), in which a polarization conversion element that includes a first prism and a second prism is provided on an optical path of light of each of the colors emitted from the red LED, the green LED and the blue LED to reduce the loss in the respective liquid-crystal panel sections and to improve light utilitization efficiency, has been considered.
Each of the first prism and the second prism is a cuboidal prism formed by bonding two right angle prisms together.
The first prism includes a polarized light splitting film formed in a surface on which the two right angle prisms are bonded together, the polarized light splitting film transmitting p-polarized light and reflecting s-polarized light, and light emitted from an LED is entered into the polarized light splitting film at an incident angle of approximately 45 degrees. A surface of the first prism that is positioned in a travelling direction of p-polarized light that has passed through the polarized light splitting film is an exit surface, and the p-polarized light is exited from the exit surface.
The second prism includes a reflective film formed in a surface on which the two right angle prisms are bonded together, and s-polarized light reflected by the polarized light splitting film in the first prism is entered into the reflective film at an incident angle of approximately 45 degree. A surface of the second prism that is positioned in a travelling direction of light reflected by the reflective film is an exit surface, and on the exit surface, a retardation plate for converting s-polarized light into p-polarized light is provided.
The p-polarized light that exits from the first prism and the p-polarized light that exits from the second prism travel in a same direction.
However, in the second approach, the area of the exit surface (the first and second exit surfaces) of the polarized light conversion element is approximately twice the light emission area of the LED. Thus, the value of the light source-side etendue is larger than the value of the image formation-side etendue, and as a result, light not used as projection light is increased, resulting in a decrease in light utilitization efficiency. As described above, because of the etendue restriction, even if a polarized light conversion element is used, the light utilitization efficiency cannot be enhanced so much and a projection image with sufficient brightness cannot be provided under the current circumstances.
As a third approach for obtaining a bright projection image, there is the method of increasing the luminous flux of light emitted from a light source without increasing the value of the light source-side etendue (see Patent Literature 1).
A three panel-type projector described in Patent Literature 1 includes a first green LED and a second green LED with different peak wavelengths, and a red LED and a blue LED.
The optical axis of the first green LED is perpendicular to the optical axis of the second green LED, and a dichroic mirror is provided at a position where the optical axes of the first and second green LEDs cross.
A green light beam emitted from the first green LED is reflected by the dichroic mirror, and the reflected light is applied to a liquid-crystal panel for green. A green light beam emitted from the second green LED passes through the dichroic mirror and the passed light is applied to the liquid-crystal panel for green.
A red light beam emitted from the red LED is applied to a liquid-crystal panel for red. A blue light beam emitted from the blue LED is applied to a liquid-crystal panel for blue.
The red image light from the liquid-crystal panel for red, the green image light from the liquid-crystal panel for green and the blue image light from the liquid-crystal panel for blue are combined by a cross dichroic prism. Image light resulting from the combination by the cross dichroic prism is projected onto a screen via a projection lens.
In the above three panel-type projector, the first green light beam emitted from the first green LED and the second green light beam emitted from the second green LED are applied to the liquid-crystal panel for green on the same optical path via the dichroic mirror. According to this configuration, since the value of the light source-side etendue is not increased, most of the first and second green light beams emitted from the first and second green LEDs can be used as projection light. Also, most of the red and blue light beams emitted from the red LED and the blue LED can be used as projection light.